The invention relates to a screw jack with a screw shaft and several ball bearing units that surround the screw eccentrically and form a nut structure, the ball bearing units engaging unilaterally into the thread and rolling in the thread and forming one part with or being joined to an inner track ring of one ball bearing unit, the outer rings of the ball bearings of the individual ball bearing units being joined with each other. Such a screw jack is described in DE-OS No. 27 25 054.
The screw jack already known has two ball bearing units in the nut structure which engage with radial pre-load from opposite sides into the thread. Since the ball bearing units are necessarily arranged sideways at a distance from each other, the radial pre-load causes a tilting moment effecting the nut structure, which is balanced by a neck bearing that also engages in the thread. This is a disadvantage with regard to the efficiency, the smoothness of running, the long life and the idling moment, particularly if the length of the screw shaft is big compared to the diameter of the screw. A rather high stress is often also caused by an improper adjustment of the ball bearing units. Such wrong adjustments are mostly only recognized by bad running and an increased wear of the neck bearings.
Attempts have been made to diminish the negative influence of the tilting moment caused by the two ball bearing units by using a ball bearing as the neck bearing. However, it turned out that such bearings did not offer a satisfactory solution in case of high stress, especially during non-vibrating working at high rotational speed.
The object of the invention is to develop a screw jack as mentioned before in such a way that an optimal functioning, in particular a very smooth running with little friction, is reached even under unfavorable conditions.
This object is achieved by providing at least three ball bearing units that engage with the thread whereby the points of engagement into the thread are all within a plane of action that is lying in the longitudinal direction of the screw shaft. At least one ball bearing unit engages at a point 180 opposed to that of the other ball bearing units, the nut structure being supported at the screw shaft solely by these points of engagement of the ball bearing units.
In such a screw jack the moments and forces caused by the radial forces of the ball bearing units compensate each other. Thus a neck bearing which supports itself on the screw shaft becomes unnecessary and consequently the screw jack can work with extremely little friction and vibration. Thanks to the arrangement according to the invention such a screw jack that necessarily works without clearance is especially adapted for high rotational speed.
The kind of ball bearings that are used is not important for the invention. One can use a variety of different arrangements of bearings (x-arrangement, O-arrangement, tandem-arrangement) to realise the invention. It is also possible to combine an axial bearing with a radial bearing to form the ball bearing unit. Such a ball bearing unit would be very small.
The screw jack is constructed in a particularly simple way if there are altogether three ball bearing units, the middle one of which engages from one side of the screw shaft into the thread and the other two engage from the opposite side of the screw shaft into the thread. Since the middle one of the ball bearing units is loaded twice as much as the two outer ball bearing units it is advisable to use a stronger ball bearing for this ball bearing unit.
In case especially high axial forces are to be transmitted it is advisable according to another embodiment of the invention to provide a total of four ball bearing units, the two middle ones of which engage from one side of the screw shaft into the thread and the two outer ones engage from the opposite side of the screw shaft into the thread. Compared to the embodiment with only three ball bearing units this embodiment has the advantage that one of the ball bearing units no longer has to cause double as much pre-load as the other ones.
If the nut structure has a rigid casing the radial pre-load of the ball bearing units can be caused simply by providing a spring which supports itself on the one hand on the middle one or ones of the ball bearing units and on the other hand on the connection of the outer rings of all ball bearing units.
The nut structure, however, does not need any separate springs at all if the outer rings are connected by at least one longitudinal member made of hardened spring steel which presses the ball bearing units as their point of engagement into the thread.
It is also advantageous in case of four or more ball bearing units that all have nearly the same radial pre-load and so nearly the same pressing force at their relevant point of engagement. Thus all ball bearing units are loaded equally so that they can be dimensioned equally.
If especially high forces are to be expected from the side that do not run in direction of the points of engagement it is advantageous according to another embodiment of the invention to provide several nut structures as described before which in view of their radial pre-load are arranged in different angular positions to each other.
The nut structure does not need a casing if it is made up of only the ball bearing units and at least one longitudinal member that joins the outer rings of the ball bearing units to each other.
According to another embodiment of the invention, the--; screw jack is adapted to transfer particularly high axial forces if, symmetrically to a line that leads radially through the screw, four ball bearing units are arranged equidistant from each other and engage in one side of the screw, and if on the opposite side two further ball bearing units engage the screw in the same plane of action on both sides of the radial line and at the same distance to it.
In a screw jack with a larger number of ball bearing units the forces on the ball bearings are nearly made equal if the ball bearing units next to each other on one side of engagement are supported on this side by a balance beam against the casing of the nut structure.
The pre-load of the individual ball bearing units can be adjusted without influencing the other ball bearing units if each ball bearing unit is fixed to at least one spring bridge that is mounted on the nut structure crosswise to the screw shaft.